Our objective continues to be the study of the regulation of immune responses. Antibody against a receptor for a single antigenic determinant, phosphorylcholine, specifically inhibits induction of the antibody response to phosphorylcholine in vitro and in vivo. Phosphorylcholine is the major antigenic determinant of the rough strain of pneumococcus, R36A. Immunization with low doses of this antigen results in production of antibody to phosphorylcholine, while immunization with high or "paralyzing" doses results in production of small amounts of antibody to phosphorylcholine and also antibody directed against the receptor for phosphorylcholine. Preliminary evidence strongly suggests that hyperimmunization of rats or mice with either of two other antigens also results in production of specific antibody and also antibody directed against the receptor for the antigen. We are presently attempting to assess the role that antibody against receptors may play in the regulation of immune responses. For these studies, we continue our previous work on the various cell types involved in immune responses that we may determine precisely where antigen, antibody, and antibody to receptors act in the regulation of responses.